Intracavity loss element for power amplifier

ABSTRACT

A regenerative ring resonator in the path of a light beam includes a discharge chamber having electrodes and a gain medium between the electrodes; an optical coupler that is partially reflective so that at least a portion of a light beam impinging on the optical coupler from the discharge chamber is reflected back through the discharge chamber and at least a portion of the light beam impinging on the optical coupler from the discharge chamber is transmitted through the optical coupler; and an attenuation optical system in the path of the light beam within the resonator, the attenuation optical system having a plurality of distinct attenuation states, with each attenuation state defining a distinct attenuation factor applied to the light beam to provide adjustment of an energy of the light beam.

TECHNICAL FIELD

The disclosed subject matter relates to an intracavity loss element fora power amplifier of a high power deep ultraviolet two-stage lightsource.

BACKGROUND

Gas discharge lasers are used in photolithography to manufacturesemiconductor integrated circuits. As semiconductor manufacturing hasprogressed to requiring smaller and smaller feature sizes (that is, theminimum feature size used to fabricate the integrated circuit), thedesign and performance of these lasers has improved. For example, gasdischarge lasers have been redesigned to provide shorter wavelength andnarrower bandwidth to support higher resolution, to provide higherpowers to enable higher throughput, and to stabilize performanceparameters such as dose, wavelength, and bandwidth.

Excimer lasers are one type of gas discharge laser used inphotolithography that can operate in the ultraviolet (UV) spectralregion at high average output power to generate nanosecond pulses atreduced spectral bandwidth.

In some cases, these lasers are designed with a dual chamber designhaving first and second chambers to separate the functions of providingnarrow spectral bandwidth and generating high average output pulseenergy. The first chamber is called a master oscillator (MO) thatprovides a seed laser beam and the second chamber is called a poweramplifier (PA) or a power oscillator (PO). If the power amplifier isdesigned as a regenerative ring resonator then it is described as apower ring amplifier (PRA). The power amplifier receives the seed laserbeam from the MO. The MO chamber enables fine tuning of parameters suchas the center wavelength and the bandwidth at relatively low outputpulse energies. The power amplifier receives the output from the masteroscillator and amplifies this output to attain the necessary powers foroutput to use in photolithography. The dual chamber design can bereferred to as a MOPA, MOPO, or MOPRA, depending on how the secondchamber is used.

SUMMARY

In some general aspects, a deep ultraviolet light source includes aregenerative ring resonator and a control system connected to theregenerative ring resonator. The regenerative ring resonator includes adischarge chamber having electrodes and a gain medium between theelectrodes; an optical coupler that is partially reflective so that atleast a portion of a light beam impinging on the optical coupler fromthe discharge chamber is reflected back through the discharge chamberand at least a portion of the light beam impinging on the opticalcoupler from the discharge chamber is transmitted through the opticalcoupler; and an attenuation optical system in the path of the light beamwithin the resonator, the attenuation optical system having a pluralityof distinct attenuation states, with each attenuation state defining adistinct attenuation factor applied to the light beam. The controlsystem is connected to the attenuation optical system and configured toselect an attenuation state applied to the light beam to thereby adjustan energy of the light beam output from the light source.

Aspects can include one or more of the following features. For example,the regenerative ring resonator can be defined by the optical couplerand a beam reverser on a side of the discharge chamber opposite to theside at which the optical coupler is facing.

The regenerative ring resonator can include a beam magnification andde-magnification stage that de-magnifies the light beam as the lightbeam travels along a first direction from the optical coupler toward thedischarge chamber and that magnifies the light beam as the light beamtravels along a second direction away from the discharge chamber towardthe optical coupler. The attenuation optical system can be between themagnification and de-magnification stage and the optical coupler. Theattenuation optical system can be inside of the magnification andde-magnification stage. The magnification and de-magnification stage caninclude a set of prisms. The prism set can include first, second, andthird prisms configured and arranged so that the first and third prismsreduce the transverse size of the profile of the light beam travellingalong the first direction through the magnification and de-magnificationstage, and the third and second prisms increase the transverse size ofthe profile of the light beam travelling along the second directionthrough the magnification and de-magnification stage.

The attenuation optical system can include a plate having at least onemesh portion through which the light beam travels, the mesh portionapplying the attenuation factor to the light beam. The mesh portion caninclude openings defined within the plate body, the size and geometry ofthe openings determine the attenuation factor of the light beam.

The attenuation factor applied to the light beam can include an amountof attenuation applied to the light beam. The attenuation amount appliedto the light beam can be a loss in intensity of the flux of the lightbeam.

The attenuation optical system can be between the optical coupler andthe discharge chamber. The attenuation optical system can include aplate including a plurality of attenuation regions with each regiondefining an attenuation factor, the plate being moveable between aplurality of positions, with each position defining an attenuationregion through which the light beam travels.

In another general aspect, a method is performed for switching betweenpulse energy ranges of a light beam output from a deep ultraviolet lightsource that includes a master oscillator providing a seed light beam toa regenerative ring resonator of a power amplifier. The method includesselecting a pulse energy range for the output light beam from among aplurality of pulse energy ranges; based on the selected pulse energyrange for the output light beam, applying a voltage to electrodes of agas discharge chamber of the master oscillator, the applied voltagebeing selected from an available voltage control range for the masteroscillator, and applying a voltage to electrodes of a gas dischargechamber of the regenerative ring resonator; operating the light sourceat the selected pulse energy range; and if it is determined that thepulse energy range of the output light beam is to be changed, selectinganother pulse energy range for the output light beam by adjusting,within the regenerative ring resonator, an attenuation of the light beamwhile maintaining a ratio of a change of applied voltage to the masteroscillator electrodes to the available voltage control range below apredetermined value.

Implementations can include one or more of the following features. Forexample, the pulse energy range can be changed while maintaining a gasrecipe of the gas discharge chamber of the power amplifier.

The attenuation of the light beam within the regenerative ring resonatorcan be adjusted by absorbing at least some of the flux of the lightbeam. The attenuation of the light beam can be adjusted by absorbing atleast 20% of the flux of the light beam.

The ratio of the change of applied voltage to the master oscillatorelectrodes to the available voltage control range can be maintainedbelow the predetermined value of 0.10.

The attenuation of the light beam within the regenerative ring resonatorcan be adjusted by adding a loss to the regenerative ring resonator.

In another general aspect, a regenerative ring resonator in the path ofa light beam includes a discharge chamber having electrodes and a gainmedium between the electrodes; an optical coupler that is partiallyreflective so that at least a portion of a light beam impinging on theoptical coupler from the discharge chamber is reflected back through thedischarge chamber and at least a portion of the light beam impinging onthe optical coupler from the discharge chamber is transmitted throughthe optical coupler; and an attenuation optical system in the path ofthe light beam within the resonator, the attenuation optical systemhaving a plurality of distinct attenuation states, with each attenuationstate defining a distinct attenuation factor applied to the light beamto provide adjustment of an energy of the light beam.

In another general aspect, a photolithography system includes alithography exposure apparatus; and a deep ultraviolet light sourcesupplying light to the lithography exposure apparatus. The light sourceincludes a regenerative ring resonator and a control system connected tothe light source. The regenerative ring resonator includes a dischargechamber having electrodes and a gain medium between the electrodes; anoptical coupler that is partially reflective so that at least a portionof a light beam impinging on the optical coupler from the dischargechamber is reflected back through the discharge chamber and at least aportion of the light beam impinging on the optical coupler from thedischarge chamber is transmitted through the optical coupler; and anattenuation optical system in the path of the light beam within theresonator, the attenuation optical system having a plurality of distinctattenuation states, with each attenuation state defining a distinctattenuation factor applied to the light beam. The control system isconnected to the attenuation optical system and is configured to selectan attenuation state applied to the light beam to thereby adjust anenergy of the light beam output from the light source.

In another general aspect, an attenuation optical system is designed foruse in a beam path of a light beam traveling through a regenerative ringresonator. The attenuation optical system includes an actuatorconfigured to receive an electromagnetic signal from a control system;and a plate mounted to the actuator to be moveable between a pluralityof positions, with each position placing an attenuation region in thebeam path such that the beam profile is covered by the attenuationregion and each attenuation region representing an attenuation factorapplied to the light beam as determined by a geometry of the attenuationregion. At least one attenuation region includes a plurality ofevenly-spaced elongated openings between solid energy-absorbing surfacesand at least one attenuation region includes an opening that is largerthan the beam profile of the light beam.

DRAWING DESCRIPTION

FIG. 1 is a block diagram of a photolithography system including a deepultraviolet light source and an intracavity attenuation optical system;

FIG. 2A is a block diagram of an exemplary power ring amplifier of thedeep ultraviolet light source of FIG. 2A;

FIG. 2B is a block diagram of an exemplary beam modification opticalsystem of the power ring amplifier of FIG. 2A;

FIGS. 3-8 are block diagrams of exemplary power ring amplifiers of thedeep ultraviolet light source of FIG. 2A;

FIGS. 9A and 9B are block diagrams of an exemplary beam modificationoptical system including an attenuation optical system;

FIG. 10A is a view of an exemplary attenuation optical system takenalong the optical axis of the light beam and showing the relativelocation of the transverse beam profile of the light beam when theattenuation optical system is in a first attenuation state;

FIG. 10B is a side view of the exemplary attenuation optical system ofFIG. 10A;

FIG. 10C is a view of the attenuation optical system of FIG. 10A takenalong the optical axis of the light beam and showing the relativelocation of the transverse beam profile of the light beam when theattenuation optical system is in a second attenuation state;

FIG. 10D is a side view of the attenuation optical system of FIG. 10C;

FIG. 11A is a view of an exemplary attenuation optical system takenalong the optical axis of the light beam and showing the relativelocation of the transverse beam profile of the light beam when theattenuation optical system is in a first attenuation state;

FIG. 11B is a side view of the attenuation optical system of FIG. 11A;

FIG. 11C is a view of the attenuation optical system of FIG. 11A takenalong the optical axis of the light beam and showing the relativelocation of the transverse beam profile of the light beam when theattenuation optical system is in a third attenuation state;

FIG. 11D is a side view of the attenuation optical system of FIG. 11C;

FIG. 11E is a view of the exemplary attenuation optical system of FIG.11A taken along the optical axis of the light beam and showing therelative location of the transverse beam profile of the light beam whenthe attenuation optical system is in a fourth attenuation state;

FIG. 11F is a side view of the attenuation optical system of FIG. 11E;

FIG. 12 is a flow chart of a procedure performed by the deep ultravioletlight source to adjust the output power of the light beam input to alithography exposure apparatus of FIG. 1.

DESCRIPTION

Referring to FIG. 1, a photolithography system 100 includes a deepultraviolet light source 110 such as an excimer light source thatoutputs a pulsed light beam 160 (which can be a laser beam) that isdirected to a lithography exposure apparatus 165. As the light beam 160enters the apparatus 165, it is directed through optics 167 that modifythe beam such as a reticle (or mask) that filters the beam, and thatmodified beam is projected onto a prepared wafer 169. In this way, achip design is patterned onto a photoresist that is then etched andcleaned, and the process repeats. The lithography exposure apparatus 165can be an immersion system or a dry system, depending on theapplication.

The system 100 also includes a control system 170 that is connected tocomponents of the light source 110 as well as the lithography exposureapparatus 165 to control various operations of the system 100.

The light source 110 can be designed as a dual chamber architecture thatincludes a master oscillator (MO) 112 that provides a seed light beam toa power amplifier (PA) 130, which can be configured as a regenerativering resonator. The master oscillator 112 enables fine tuning ofparameters such as the center wavelength and the bandwidth at relativelylow output pulse energies. The power amplifier 130 receives the outputfrom the master oscillator 112 and amplifies the output to attain thenecessary powers in the light beam 160 for output to use in thelithography apparatus 165.

The master oscillator 112 includes a discharge chamber 114 having twoelongated electrodes 115, a gain medium that is a gas mixture, and a fanfor circulating gas between the electrodes 115. A resonator is formedbetween a line narrowing module 116 on one side of the discharge chamber114 and an output coupler 118 on a second side of the discharge chamber114. The line narrowing module 116 can include a diffractive optic suchas a grating that finely tunes the spectral output of the dischargechamber 114. The master oscillator 112 also includes a line centeranalysis module 120 that receives an output from the output coupler 118and a beam modification optical system 122 that modifies the size orshape of the light beam as needed to form the seed light beam 124. Thegas mixture used in the discharge chamber 114 can be any suitable gasfor producing a light beam at the required wavelength (ultraviolet) andbandwidth. For example, for an excimer source, the gas mixture typicallycontains a noble gas (rare gas) (for example, argon, krypton, or xenon)and a halogen (for example, fluorine or chlorine), apart from heliumand/or neon as buffer gas. Specific examples of the gas mixture includeargon fluoride (ArF), which emits light at a wavelength of about 193 nm,krypton fluoride (KrF), which emits light at a wavelength of about 248nm, or xenon chloride (XeCl), which emits light at a wavelength of about351 nm. The excimer gain medium (the gas mixture) is pumped with short(for example, nanosecond) current pulses in a high-voltage electricdischarge by application of a voltage to the elongated electrodes 115.

The power amplifier 130 includes a beam modification optical system 132that receives the seed light beam 124 from the master oscillator 112 anddirects the light beam through a discharge chamber 140, and to a beamturning optical element 150 where the direction of the light beam ismodified so that it is sent back into the discharge chamber 140. If thepower amplifier 130 is designed as a regenerative ring resonator, thenthe light beam is directed through the power amplifier 130 to form acirculating path.

The discharge chamber 140 includes a pair of elongated electrodes 141, again medium that is a gas mixture, and a fan for circulating the gasmixture between the electrodes 141. In the regenerative ring resonatordesign, the seed light beam 124 is amplified by repeatedly passingthrough the discharge chamber 140. The optical system 132 provides a way(for example, an optical coupler such as a partially-reflecting mirror,discussed below) to in-couple the seed light beam 124 and to out-couplea portion of the amplified radiation from the ring resonator to form theoutput light beam 160. The output light beam 160 is directed through abandwidth analysis module 162, where various parameters of the beam 160can be measured. The output light beam 160 can also be directed througha pulse stretcher, where each of the pulses of the output light beam 160is stretched, for example, in an optical delay unit, to adjust forperformance properties of the light beam that impinges the lithographyapparatus 165.

The power amplifier 130 also includes an attenuation optical system 152in the path of the light beam within the resonator defined in theamplifier 130. As discussed in greater detail below, the attenuationoptical system 152 has a plurality of distinct attenuation states, witheach attenuation state defining a distinct attenuation factor applied tothe light beam 160.

The system 152 is “in the path” of the light beam in that it is eitherdirectly in the path of the light beam so that the beam profile impingesupon matter of the system 152 or it is adjacent to the path of the lightbeam so that the beam profile freely passes through the system 152 ornext to the system 152 without touching the matter of the system 152.Various components of the system 152 may be moved, for example, rotatedabout the optic axis of the light beam or about an axis that isperpendicular to the optic axis, or translated along a linear path thatis perpendicular to the optic axis of the light beam, as discussedbelow, to adjust the attenuation factor applied to the light beam 160.Thus, the attenuation optical system 152 can also include one or moreactuators for moving components of the system 152 to provide for theadjustment. The adjustment of the attenuation factor applied to thelight beam 160 can be in a step-wise or discrete manner or it can be ina continuous manner.

The pulse energies output from the master oscillator 112 and the poweramplifier 130 are determined by the respective gains applied and thelosses (which cause a loss in intensity of the beam flux through thegain medium) within the respective chambers. The gain in the masteroscillator and the gain in the power amplifier are determined by therespective operating voltages, that is, the voltage applied to therespective discharge electrodes. Thus, the pulse energy output from thepower amplifier 130 can be adjusted by adjusting the losses within thepower amplifier 130 without having to adjust the operating voltage ofeither the master oscillator 112 or the power amplifier 130. Theattenuation optical system 152 is therefore provided within the poweramplifier 130 to adjust the loss in the power amplifier 130 to therebyadjust the pulse energy of the light beam 160 output from the poweramplifier 130 without requiring an adjustment to the operating voltagesand without requiring other adjustments to the gain medium (for example,without requiring a gas refill with a different fill pressure to adjustthe gain). Additionally, because the attenuation is applied to the lightbeam 124 in the power amplifier 130, the master oscillator 112 cavityloss is not modified, and because the operating voltage of the masteroscillator 112 remains within an acceptable level, the master oscillator112 output pulse energy does not change appreciably.

The control system 170 is connected to various components of the lightsource 110. For example, the control system 170 is coupled to theelectrodes 115, 141 within the master oscillator 112 and the poweramplifier 130, respectively, for controlling the respective pulseenergies of the master oscillator 112 and the power amplifier 130, andalso for controlling the pulse repetition rates, which can range betweenabout 1000 and 12,000 Hz or greater. The control system 170 thereforeprovides repetitive triggering of the discharges in the chamber of themaster oscillator 112 and the discharges in the chamber of the poweramplifier 130 relative to each other with feedback and feed-forwardcontrol of the pulse and dose energy. The repetitively-pulsed light beam160 can have an average output power of between a few watts and hundredsof watts, for example, from about 40 W to about 200 W. The irradiance(that is, the average power per unit area) of the light beam 160 at theoutput can be at least about 60 W/cm² or at least about 80 W/cm².

Typically, the output power of the light source 110 is calculated at100% duty cycle (that is, the continuous firing of the electrodes in themaster oscillator 112 and the power amplifier 130 of the light source110) at a nominal pulse repetition rate and a nominal pulse energy.Thus, for example, at a nominal pulse repetition rate of 6000 Hz and a15 mJ nominal pulse energy, the output power of the light source 110(which is the power of the light beam 160) is 90 W. As another example,at a nominal pulse repetition rate of 6000 Hz and a 20 mJ nominal pulseenergy, the output power of the light source 110 (which is the power ofthe light beam 160) is 120 W.

The control system 170 is connected to the actuators of the attenuationoptical system 152 to move components of the attenuation optical system152 to adjust the loss within the power amplifier 130 (by modifying theattenuation factor applied to the light beam), and to thereby adjust theaverage output power of the light beam 160.

As discussed above, the attenuation is applied to the light beam 160while maintaining the operating voltage of the master oscillator 112within an acceptable level. The operating voltage is the voltage appliedto the master oscillator 112; and it is possible that the voltageapplied to the electrodes 115 of the master oscillator 112 will changewhen the attenuation applied to the light beam 160 is adjusted. However,the ratio of the change in this applied voltage to an available voltagecontrol range is kept below a predetermined acceptable value. Forexample, the operating voltage may change by as much as about 20 V whenan adjustment is made to the attenuation factor; while the availablevoltage control range is about 200 V; in this example, the ratio is ashigh as 0.10.

Referring to FIGS. 2A and 2B, an exemplary power ring amplifier 130 isshown in which the attenuation optical system 152 is placed in a genericlocation within power ring amplifier 130. Specific exemplary placementsfor the attenuation optical system 152 are described and shown in FIGS.3-8.

In these examples, the power ring amplifier 130 is designed as aregenerative ring resonator. In such a design, a standard tilteddouble-pass optical path through the discharge chamber 140 is closedwith the use of an optical coupler 234 to form a recirculating resonantstructure that enables regenerative amplification of the seed light beampulse from the master oscillator 112. The seed light beam 124 from themaster oscillator 112 is directed through the optical coupler 234, whichis a partially reflecting mirror (and can be referred to as aninput/output coupler) and serves as both the entrance into the ringresonator and also the exit from the ring resonator. The optical coupler234 can have a reflectivity of between about 10% to about 60%; to forman oscillation cavity that allows the pulse intensity to build up duringthe oscillation through the excited gain medium in the discharge chamber140 between the electrodes during the electrical discharge.

The light beam that travels through the optical coupler 234 is reflectedfrom a reflecting optic such as a mirror 236 toward the dischargechamber 140. The mirror 236 can have a high reflectivity, for examplegreater than about 90% at or near the center wavelength of the lightbeam for the desired polarization at the angle of incidence used. Thelight beam reflected from the mirror is directed through the dischargechamber 140 and toward the beam turning optical element 150, which inthis implementation is a prism 254. The light beam reflected from twosurfaces of the prism 254 reenters the discharge chamber 140 alonganother path and back toward the optical coupler 234, where, asdiscussed above, some of the light beam is reflected back into the ringresonator while some of the light beam is transmitted through theoptical coupler 234 as the output light beam 160.

The beam turning optical system 150 is an optical system made of one ormore precision devices having precision optical materials such asmaterials having a crystalline structure such as calcium fluoride (CaF2)and also includes precision optically finished faces. Although a prism254 is shown in these examples, the beam turning optical system 150 canbe any combination of one or more optical devices that receive the lightbeam and change the direction of the light beam so that it istransmitted back into the discharge chamber 140. Thus, in otherimplementations, the beam turning optical system 150 can include aplurality of mirrors arranged to reflect the light beam back into thedischarge chamber 140.

Additionally, the light beam that is reflected from the mirror 236through the discharge chamber 140 is usually compressed before enteringthe discharge chamber 140 so that it can substantially match thetransverse size of the gain medium in the discharge chamber 140. If thelight beam is compressed before entering the discharge chamber 140, thenit is also expanded as it exits the discharge chamber 140. In order toperform the compression and expansion, a beammagnification/de-magnification system 238 is positioned between thedischarge chamber 140 and the mirror 236 and optical coupler 234. Thebeam magnification/de-magnification system 238 can include any number ofoptical elements such as prisms to perform the compression and expansionof the beam.

One particular implementation of the beam magnification/de-magnificationsystem 238 is shown in FIG. 2B. This system 238 includes first prism242, second prism 244, and third prism 246. The first prism 242 and thethird prism 246 act together to compress the incoming light beam 124,and the third prism 246 also aligns the incoming light beam with bothwindows of the discharge chamber 140. The third prism 246 shifts theoutgoing light beam 140 (which has been reflected by the prism 254 backthrough the discharge chamber 140) to the second prism 244, which shiftsthe light beam to the optical coupler 234. The third prism 246 and thesecond prism 244 act together to magnify or expand the outgoing lightbeam, the one that exits the discharge chamber 140 toward the opticalcoupler 234, to match the transverse size of the incoming light beam124. The outgoing light beam impinges upon the optical coupler 234,where it is either transmitted through to form the light beam 160 or itis reflected back into the ring resonator.

All of the optical components (such as the optical coupler 234, themirror 236, the prisms 242, 244, 246, 254, and the chamber windows) aretypically crystalline structures that are able to transmit very highpulse energy light or laser pulses at very short wavelengths (deepultraviolet wavelengths) with minimal losses. For example, thecomponents can be made of calcium fluoride (CaF2), magnesium fluoride(MgF2), or fused silica.

The following discussion uses the terms “beam profile,” “near field,”and “far field” to describe some of the optical effects noticed withinthe power ring amplifier 130. The term “beam profile” is thedistribution of energy in position across a direction that is transverseto the beam propagation direction. The “near field” beam profile refersto the distribution of electromagnetic energy in the vicinity of anobject (for example, an aperture or a mask) that changes a shape of thebeam. The “far field” beam profile is the distribution ofelectromagnetic energy far away from the object.

Referring to FIGS. 3-8, in one implementation, the attenuation opticalsystem 152 is shown placed at various possible locations throughout thepower ring amplifier 130. For example, in FIG. 3, the attenuationoptical system 152 is placed inside the beam modification optical system132 in the path of the light beam as it travels from the mirror 236toward the beam magnification/de-magnification system 238. In FIG. 4,the attenuation optical system 152 is placed inside the beammodification optical system 132 in the path of the light beam as ittravels from the beam magnification/de-magnification system 238 towardthe optical coupler 234. In FIG. 5, the attenuation optical system 152is placed inside the beam modification optical system 132 in the path ofthe light beam as it travels from the discharge chamber 140 toward thebeam magnification/de-magnification system 238. In FIG. 6, theattenuation optical system 152 is placed in the path of the light beamas it travels from the discharge chamber 140 toward the prism 254. InFIG. 7, the attenuation optical system 152 is placed in the path of thelight beam as it travels from the beam magnification/de-magnificationsystem 238 toward the discharge chamber 140. In FIG. 8, the attenuationoptical system 152 is placed in the path of the light beam as it travelsthrough the beam magnification/de-magnification system 238 toward thedischarge chamber 140.

Referring to FIGS. 9A and 9B, in an exemplary implementation, theattenuation optical system 152 can be placed inside the beammagnification/de-magnification system 238. In this case, it is in thepath of the light beam as it travels from the first prism 242 toward thethird prism 246. As shown in FIG. 9A, the attenuation optical system 152is set up with a first attenuation state in which the entire light beamis permitted to pass from the first prism 242 toward the third prism 246without suffering any loss or attenuation. As discussed below, theattenuation factor applied to the light beam is one (1). As shown inFIG. 9B, the attenuation optical system 152 is set up with a secondattenuation state, in which the light beam suffers a loss as it travelsfrom the first prism 242 toward the third prism 246 by traveling througha portion of the system 152 that absorbs at least some of the flux,diffusely reflects at least some of flux, or both, of the light beam.

The attenuation optical system 152 includes an actuator 953 configuredto receive an electromagnetic signal from the control system 170, and aplate 954 mounted to the actuator to be moveable between a plurality ofpositions. In this case, it is moveable between two positions. Eachposition places an attenuation region in the beam path such that thebeam profile is covered by the attenuation region and each attenuationregion represents an attenuation factor applied to the light beam asdetermined by a geometry of the attenuation region. At least oneattenuation region includes a plurality of evenly-spaced elongatedopenings between solid energy-absorbing surfaces and at least oneattenuation region includes an opening that is larger than the beamprofile of the light beam.

In general, the attenuation factor is determined by the material of thesystem 152, and the geometry and placement of the system 152 relative tothe light beam.

An exemplary design for an attenuation optical system 1052 is shown inFIGS. 10A-10D. In this design, the system 1052 includes a solid plate1054 having a plurality of through openings 1056 through which some ofthe light beam can pass when the plate 1054 is inserted directly intothe path 1058 of the light beam.

In the system 1052, the through openings 1056 have an elongatedrectangular geometry. However, other geometries are possible, asdiscussed below. The geometry of the openings 1056 can be selected orconfigured to provide for specific attenuation factors. For example, theattenuation factor applied to the light beam can be determined to be theratio of a total open area to a total area at which the light beamimpinges. The total open area is the total area of the openings 1056taken along the plane transverse to the light beam optic axis (which isalong the direction of the path 1058) that is covered by the beamprofile 1062 and the total area is the total area of both the open areaand the solid surface that is covered by the beam profile 1062. Thus,the attenuation factor can be adjusted by adjusting the relative sizesof the open area to the solid area, which is directly dependent on thegeometry of the plate 1054. For example, if the light beam passesthrough the attenuation optical system 1052 without any loss, then thetotal open area is equal to the total area covered by the beam profile1062 and therefore the attenuation factor is 1. As another example, ifthe total open area is half the total area covered by the beam profile1062, then the attenuation factor is 0.5. Thus, the attenuation factorranges from 0 to 1, with 0 being 100% loss added to the light beam (inthis case, the light beam is completely blocked) and 1 being 0% lossadded to the light beam (in this case, the light beam passes entirelythrough without any loss).

There are two distinct attenuation states associated with the system1052. In the first state, which is shown in FIGS. 10A and 10B, there isno attenuation and an attenuation factor of 1 is applied to the lightbeam. The plate 1054 is entirely out of the path 1058 of the light beamso that the entire light beam profile 1062 passes along the path 1058unimpeded through the power ring amplifier 130. In the second state,which is shown in FIGS. 10C and 10D, there is an attenuation factor ofabout 0.5 applied to the light beam. The plate 1054, and specificallythe portion with the openings 1056 is placed in the path of the lightbeam so that the beam profile 1062 of the light beam impinges upon theplate 1054.

Additionally, the geometry of the openings 1056 can be selected toimprove certain characteristics of the beam profile. Thus, for example,elongated rectangular openings such as those shown in FIGS. 10A-10D maybe suitable for maintaining specifications of the near field beamparameters of the light beam or to reduce the effect of beam modulationon the far field profile of the light beam. The actual geometry selecteddepends on the beam profile properties.

The openings 1056 can be arranged to extend beyond the beam profile 1062to ensure that the entire beam profile 1062 is covered. Additionally,the arrangement and shape of the openings 1056 can be selected toprovide a uniform geometric distribution across the beam profile 1062without additional modulation to the plane of the beam. However, it maybe possible to arrange the openings 1056 in a non-uniform or even randommanner, depending on the application and the characteristics of the beamprofile 1062.

The material of the plate 1054 is selected based on several factors. Onefactor to consider when selecting the material is the ability of thematerial to absorb the energy or flux of the light beam. At the sametime, other factors that can be considered are the reduction ofnon-diffuse reflections from the surface and a low emissivity. Thesefactors can be important for thermal stability of the plate 1054. Forexample, tungsten is a material that is suitable for providing good heattransfer, low emissivity, high absorption, and lower amounts ofnon-diffuse reflections.

If thermal stability is a problem with the attenuation optical system1052 and 152, then it is possible to provide a dedicated heat flow pathfrom the plate to a heat sink.

Another exemplary design for an attenuation optical system 1152 is shownin FIGS. 11A-11F. In this design, the system 1152 includes a circularand rotatable plate 1154 that defines four distinct attenuation statesS1, S2, S3, S4. Each state is defined by a region of the plate 1154,which can be selectively moved by rotation of the plate 1154 about thecenter axis 1164 (which is parallel with the beam path 1158) tointersect the path 1158, with the region selected depending on thedesired attenuation factor to be applied to the light beam.

The first attenuation state S1 is shown in FIGS. 11A and 11B; in thisstate, there is no attenuation because an attenuation factor of 1 isapplied to the light beam. The region of the plate 1154 that is in thepath 1158 of the beam profile 1162 is entirely open and thus, the entirelight beam profile 1162 passes along the path 1158 unimpeded through thepower ring amplifier 130 in the first attenuation state.

The remaining attenuation states S2, S3, S4 are defined by solid regionsof the plate with a plurality of through openings, with each of thestates S2, S3, S4 having a distinct ratio of the total open area to thetotal area at which the light beam impinges. For example, in FIGS. 11Cand 11D, the third attenuation state S3 is selected, and the attenuationfactor of 0.5 associated with the state S3 is applied to the light beam.The plate 1154, and specifically the region with the openings 1156 thatdefine the third state S3 is placed in the path of the light beam sothat the beam profile 1162 of the light beam impinges upon the plate1154 in that region.

As another example, in FIGS. 11E and 11F, the fourth attenuation stateS4 is selected, and the attenuation factor of 0.7 associated with thestate S4 (which is distinct from the attenuation factor of the otherattenuation states) is applied to the light beam.

In the implementations described, the attenuation optical system 152 isa solid plate with machined through holes or openings. However, otherdesigns are possible. For example, in other implementations, theattenuation optical system 152 can include a mesh structure thatconsists of a semi-permeable barrier made of connected strands ofelongated material such as metal. The mesh structure can be similar to anet structure and have woven strands.

In other implementations, the attenuation optical system 152 can bedesigned to diffusely refract portions of the light beam to therebyincrease the loss in the power ring amplifier. In other implementations,the attenuation optical system 152 can be designed to reflect away apercentage of the light beam and direct the reflected light to a beamdump.

Other components of the power ring amplifier 130 provide innateattenuation to the light beam; however, the attenuation optical system152 provides for adjustable attenuation to enable the adjustment of thepulse energy without having to adjust the operating voltage or the gaspressure.

Referring to FIG. 12, a flow chart of a procedure 1280 is performed bythe system 100 to switch between pulse energy ranges of the light beamoutput from the light source 110. The pulse energy range for the outputlight beam is selected from among a plurality of pulse energy ranges(step 1281). Based on the selected pulse energy range for the outputlight beam, the control system 170 applies a voltage to the electrodesof the gas discharge chamber 114 of the master oscillator 112 andapplies a voltage to the electrodes of the gas discharge chamber 140 ofthe regenerative ring resonator of the power ring amplifier 130 (step1282). The applied voltage is selected from an available voltage controlrange for the master oscillator 112. The light source 110 is operated atthe selected pulse energy range (step 1283). Next, if no instructionshave been received to change the pulse energy range (step 1284), thenthe light source 110 is maintained at the selected pulse energy range(step 1283). However, if instructions are received to change the pulseenergy range (step 1284) to another pulse energy range, then the controlsystem 170 sends a signal to the attenuation optical system 152 toadjust, within the regenerative ring resonator, an attenuation of thelight beam while maintaining a ratio of a change of applied voltage tothe master oscillator electrodes to the available voltage control rangebelow a predetermined value (step 1285).

Additionally, the pulse energy range can be changed at step 1285 whilemaintaining a gas recipe of the gas discharge chamber 140 of the powerring amplifier 130.

Attenuation of the light beam can be adjusted from an attenuation factorof 1 (with no attenuation) to an attenuation factor of 0.8 by absorbingabout 20% of the flux of the light beam.

Other implementations are within the scope of the following claims.

For example, while distinct attenuation factors were discussed anddescribed above, it is possible to adjust the attenuation of the lightbeam continuously so that the attenuation factor is a continuouslyvariable value applied to the light beam. Such a continuous adjustmentcan be performed by tuning an angle of the plate about an axis that isperpendicular to the optic axis (or the beam path) of the light beam. Inother implementations, the openings in the plate can be polygonal shapesor circular or oval shapes. Other metals or alloys can be used for theplate or mesh, as long as the material is able to absorb the energy ofthe light beam without overheating or changing state.

What is claimed is:
 1. A deep ultraviolet light source comprising: aregenerative ring resonator comprising: a discharge chamber havingelectrodes and a gain medium between the electrodes; an optical couplerthat is partially reflective so that at least a portion of a light beamimpinging on the optical coupler from the discharge chamber is reflectedback through the discharge chamber and at least a portion of the lightbeam impinging on the optical coupler from the discharge chamber istransmitted through the optical coupler; and an attenuation opticalsystem in the path of the light beam within the resonator, theattenuation optical system having a plurality of distinct attenuationstates, with each attenuation state defining a distinct attenuationfactor applied to the light beam; and a control system connected to theattenuation optical system and configured to select an attenuation stateapplied to the light beam to thereby adjust an energy of the light beamoutput from the light source.
 2. The light source of claim 1, whereinthe regenerative ring resonator is defined by the optical coupler and abeam reverser on a side of the discharge chamber opposite to the side atwhich the optical coupler is facing.
 3. The light source of claim 1,wherein the regenerative ring resonator comprises a beam magnificationand de-magnification stage that de-magnifies the light beam as the lightbeam travels along a first direction from the optical coupler toward thedischarge chamber and that magnifies the light beam as the light beamtravels along a second direction away from the discharge chamber towardthe optical coupler.
 4. The light source of claim 3, wherein theattenuation optical system is between the magnification andde-magnification stage and the optical coupler.
 5. The light source ofclaim 3, wherein the attenuation optical system is inside of themagnification and de-magnification stage.
 6. The light source of claim3, wherein the magnification and de-magnification stage comprises a setof prisms.
 7. The light source of claim 6, wherein the prism setcomprises first, second, and third prisms configured and arranged sothat the first and third prisms reduce the transverse size of theprofile of the light beam travelling along the first direction throughthe magnification and de-magnification stage, and the third and secondprisms increase the transverse size of the profile of the light beamtravelling along the second direction through the magnification andde-magnification stage.
 8. The light source of claim 1, wherein theattenuation optical system includes a plate having at least one meshportion through which the light beam travels, the mesh portion applyingthe attenuation factor to the light beam.
 9. The light source of claim8, wherein the mesh portion includes openings defined within the platebody, the size and geometry of the openings determine the attenuationfactor of the light beam.
 10. The light source of claim 1, wherein theattenuation factor applied to the light beam includes an amount ofattenuation applied to the light beam.
 11. The light source of claim 10,wherein the attenuation amount applied to the light beam is a loss inintensity of the flux of the light beam.
 12. The light source of claim1, wherein the attenuation optical system is between the optical couplerand the discharge chamber.
 13. The light source of claim 1, wherein theattenuation optical system comprises plate comprising a plurality ofattenuation regions with each region defining an attenuation factor, theplate being moveable between a plurality of positions, with eachposition defining an attenuation region through which the light beamtravels.
 14. A method of switching between pulse energy ranges of alight beam output from a deep ultraviolet light source that comprises amaster oscillator providing a seed light beam to a regenerative ringresonator of a power amplifier, the method comprising: selecting a pulseenergy range for the output light beam from among a plurality of pulseenergy ranges; based on the selected pulse energy range for the outputlight beam, applying a voltage to electrodes of a gas discharge chamberof the master oscillator, the applied voltage being selected from anavailable voltage control range for the master oscillator, and applyinga voltage to electrodes of a gas discharge chamber of the regenerativering resonator; operating the light source at the selected pulse energyrange; and if it is determined that the pulse energy range of the outputlight beam is to be changed, selecting another pulse energy range forthe output light beam by adjusting, within the regenerative ringresonator, an attenuation of the light beam while maintaining a ratio ofa change of applied voltage to the master oscillator electrodes to theavailable voltage control range below a predetermined value.
 15. Themethod of claim 14, wherein changing the pulse energy range includesdoing so while maintaining a gas recipe of the gas discharge chamber ofthe power amplifier.
 16. The method of claim 14, wherein adjusting theattenuation of the light beam within the regenerative ring resonatorcomprises absorbing at least some of the flux of the light beam.
 17. Themethod of claim 16, wherein adjusting the attenuation of the light beamcomprises absorbing at least 20% of the flux of the light beam.
 18. Themethod of claim 14, wherein maintaining the ratio of the change ofapplied voltage to the master oscillator electrodes to the availablevoltage control range below the predetermined value comprisesmaintaining the ratio below 0.10.
 19. The method of claim 14, whereinadjusting the attenuation of the light beam within the regenerative ringresonator comprises adding a loss to the regenerative ring resonator.20. A regenerative ring resonator in the path of a light beam, theresonator comprising: a discharge chamber having electrodes and a gainmedium between the electrodes; an optical coupler that is partiallyreflective so that at least a portion of a light beam impinging on theoptical coupler from the discharge chamber is reflected back through thedischarge chamber and at least a portion of the light beam impinging onthe optical coupler from the discharge chamber is transmitted throughthe optical coupler; and an attenuation optical system in the path ofthe light beam within the resonator, the attenuation optical systemhaving a plurality of distinct attenuation states, with each attenuationstate defining a distinct attenuation factor applied to the light beamto provide adjustment of an energy of the light beam.
 21. Aphotolithography system comprising: a lithography exposure apparatus;and a deep ultraviolet light source supplying light to the lithographyexposure apparatus, the light source comprising: a regenerative ringresonator comprising a discharge chamber having electrodes and a gainmedium between the electrodes; an optical coupler that is partiallyreflective so that at least a portion of a light beam impinging on theoptical coupler from the discharge chamber is reflected back through thedischarge chamber and at least a portion of the light beam impinging onthe optical coupler from the discharge chamber is transmitted throughthe optical coupler; and an attenuation optical system in the path ofthe light beam within the resonator, the attenuation optical systemhaving a plurality of distinct attenuation states, with each attenuationstate defining a distinct attenuation factor applied to the light beam;and a control system connected to the attenuation optical system andconfigured to select an attenuation state applied to the light beam tothereby adjust an energy of the light beam output from the light source.22. An attenuation optical system for use in a beam path of a light beamtraveling through a regenerative ring resonator, the system comprising:an actuator configured to receive an electromagnetic signal; and a platemounted to the actuator to be moveable between a plurality of positions,with each position placing an attenuation region in the beam path suchthat the beam profile is covered by the attenuation region and eachattenuation region representing an attenuation factor applied to thelight beam as determined by a geometry of the attenuation region,wherein at least one attenuation region includes a plurality ofevenly-spaced elongated openings between solid energy-absorbing surfacesand at least one attenuation region includes an opening that is largerthan the beam profile of the light beam.